The present invention relates to cobalt titanate particles and a process for producing such particles. More particularly, the present invention relates to the cobalt titanate particles which are mono-dispersed and have uniform particle size distribution, and a process for producing such particles.
A major use for the cobalt titanate particles of the present invention is in preparation of green pigments for coating materials and toner.
Recently, necessity is rising for the improvement of dispersibility of pigment particles in vehicle, their workability and thermal stability in the production of coating material so as to meet the requirement for enhancement of working efficiency in the age of energy saving and improvement of physical properties of the coats.
For the improvement of dispersibility and workability of pigment particles, it is essential that the said particles have a proper size and also uniform particle size distribution, and are mono-dispersed.
As for thermal stability, since the toner for development, whose demand is increasing recently with diffusion of copying machines, is heated to a high temperature above 150.degree. C. in its production process, the pigment particles used as colorant for such toner are required to keep stable in color even at high temperatures above 150.degree. C.
As green pigment particles, chrome green, zinc green and chrome oxide particles have been popularly used.
Also, as the green particles, cobalt titanate particles which is an ilmenite type oxide, are known. For the synthesis of such cobalt titanate particles, there is known, for instance, a method in which a mixture of CoO particles and TiO.sub.2 particles is calcined at a high temperature as described in Journal of Physical Society of Japan, Vol. 13, No. 10, pp. 1110-1118, 1958.
The pigment particles with excellent dispersibility, workability and thermal stability are now the most acutely required. Any of the hitherto known green pigment particles has some problem or other and is unable to meet all the requirements. For instance, the chrome green is a mixture of prussian blue and chrome yellow, so that when a coating material is prepared therefrom, there is a possibility that both pigments might separate from each other. Also, the zinc green is a mixture of zinc yellow and prussian blue, and when a coating material is made therefrom, there might cause separation of both pigments. The chrome oxide is obscure green and lacks vividness. Further, each of the said chrome green, zinc green and chrome oxide, because of use of noxious chrome, has difficulties in handling both in the production process and in the disposal of the pigments, and is poor in workability.
Cobalt titanate particles are excellent in thermal stability, but since their production process involves a firing step at a high temperature and a pulverization step, there takes place sintering between the particles in the course of the production process and the obtained particles have irregular shape and non-uniform particle size distribution.
So, strong request has been voiced for such cobalt titanate particles which are harmless, can be used as a green pigment without mixing two or more different types of pigment, are free from sintering between particles, are mono-dispersed and have also uniform particle size distribution.
As a result of assiduous studies for obtaining the cobalt titanate particles without conducting high-temperature firing and pulverization, it has been found that by subjecting an alkaline suspension (pH 8.0-10.0) containing Co (II) and Ti (III) to an autclaving treatment at a temperature of 250.degree.-300.degree. C., CoTiO.sub.3 particles which suffer from no sintering between particles, are mono-dispersed (discrete from each other) and have uniform particle size distribution, can be obtained. The present invention was attained on the basis of this finding.